The present invention relates to a protective device for an induction heating apparatus used for cooking by utilizing heat from electromagnetic induction.
The induction heating apparatus utilizing low frequency comprises a heating element such as a specific cooking pot and an excitor for heating the specific cooking pot. The excitor includes at least two groups of magnetic circuits and a capacitor having a specific value connected in series to one group of excitation windings so as to pass a current having a predetermined phase difference from the current passing through the other group of excitation windings to decrease noise and vibration of the cooking pot.
In comparison with the operation of the induction heating apparatus with the specific cooking pot on the excitor, an equivalent resistance of the magnetic circuit is lower during operation without the specific cooking pot. Accordingly, both the absolute value and the phase of the current passing through the magnetic circuit are changed.
In this case, the current passing through the capacitor is increased and the phase is a leading one. A similar result occurs when an undesirable cooking pot is placed on the excitor. Even though the specific cooking pot is placed on the excitor in a normal state, if the pot is heated to a very high temperature such as heating the pot when it is empty, the terminal voltage of the capacitor is also changed.
The inventors have studied and proposed protective device for overcoming these difficulties and for preventing an accident during an abnormal condition. This protective device is disclosed in U.S. Pat. No. 3,973,105, issued Aug. 3, 1976.
In accordance with the protective device, a change of the terminal voltage of the capacitor in an abnormal state is detected by the detecting circuit and the relay of the breaking drive circuit is operated by the output of the detecting circuit to actuate the relay of the breaking circuit whereby the operating voltage applied to the excitor is cut off.
However, in the protective device disclosed in the prior applications, the power voltage of the operating voltage is applied directly to the excitation windings of the excitor. Accordingly, if a cooking pot user places the cooking pot above the excitor when the power voltage is applied, a high attractive force is applied suddenly to the cooking pot. This can cause the cooking pot to be dropped by a startled user. This is especially disadvantageous when the cooking pot contains the food to be cooked.